The present invention relates to tubular retainer elements which are used to hold mine roof expansion anchors in assembled relation prior to installation in a bore hole, and to methods of using such retainer elements in combination with an expansion anchor. More specifically, the invention relates to novel configurations and methods of use of tubular retainer elements which ensure automatic removal of the retainer element during normal installation of an expansion anchor in a bore hole in a mine roof, or the like.
One of the most common means of reinforcing and supporting mine roofs and other rock structures for many years has been an expansion anchor on a threaded portion of an elongated bolt or rod installed in a preformed bore hole in the rock structure. The expansion anchor includes a tapered nut or camming plug having an internally threaded bore engaged with the threaded portion of the bolt, and an expansible shell portion which is moved radially outwardly into gripping engagement with the wall of the bore hole by axial movement of the camming plug in response to rotation of the bolt.
The expansion shell, in some designs, includes two, or sometimes more, leaf portions which are joined by a strap or bail member having end portions affixed to the leaves and a medial portion extending over the end of the camming plug, which is maintained by the bail in assembled relation with the leaves. Since the bail is normally formed from a strip of somewhat springy sheet metal, the leaf portions may be urged outwardly, away from one another by the biasing force of the bail. While such action is desirable once the assembly is inserted into the bore hole, it is to be avoided prior to such insertion since it may lead to disassembly of the plug and shell. Accordingly, it has been common practice for many years to place a tubular retainer element, normally formed of a suitable plastic, in encircling relation to the shell leaves, thus holding them in the desired, cylindrical configuration.
In order to ensure proper expansion of the shell leaves within the bore hole it is necessary to remove the plastic retainer, preferably just prior to insertion of the assembly into the bore hole. The outside diameter of prior art tubular retainers has conventionally been very close to (e.g., within a few thousandths of an inch) the nominal diameter of the bore hole; thus, manual removal of the retainer has been preferred. Although in most cases bolting crews are instructed to remove the retainer from the expansion shell prior to insertion into the bore hole, errors inevitably occur when relying upon the human element and removal of the retainer is sometimes neglected. The result of the failure to remove the retainer element is often improper expansion of the leaves, causing an incomplete or improper installation of the anchor. In some expansion shell designs, the tubular retainer may be left on as the assembly is placed in the bore hole with the intention that expansion of the leaves will break the retainer. However, in such cases, there is always the possibility of malfunction due to failure of the retainer to break in the manner intended.
Some tubular retainer elements have been formed with an outwardly extending flange at the lower end. Since the cross-sectional configuration of such elements is not uniform throughout their length, they are normally injection molded, thus being more costly to fabricate than an extruded retainer of uniform, unvarying cross section. Therefore, it is desireable to have a retainer which may be economically fabricated as well as automatically removed from encircling relation to the expansion anchor in the course of normal installation.
It is a principal object of the present invention to provide a mine roof expansion anchor including a camming plug, expansion leaves joined by a bail, and a cylindrical retainer element encircling the leaves, wherein the configuration of the retainer element is such that it is automatically removed from encircling relation with the leaves upon insertion of the assembly into a drill hole of predetermined diameter.
Another object is to provide a tubular retainer element of uniform cross-dimension thoroughout its length which is automatically removed from an expansion anchor assembly during normal installation.
A further object is to provide a method of installing an expansion anchor assembly carried on an elongated mine roof bolt or rod into a drill hole of predetermined diameter in a manner which ensures that a cylindrical retainer element which initially encircles the leaf members of the expansion shell is removed as the assembly is installed.
Other objects will in part be obvious and will in part appear hereinafter.